An inverter converts direct current (DC) to alternating current (AC) by using high-speed electronic switches—typically MOSFETs or IGBTs—arranged in an H-bridge configuration. These switches rapidly alternate the DC voltage polarity to create a square wave, then use Pulse Width Modulation (PWM) and an LC low-pass filter to smooth that chopped waveform into a clean 60 Hz (or 50 Hz) pure sine wave. If you are building an off-grid or backup power system, understanding this conversion block is only the first step. The real challenge is sizing the DC source, the wiring, and the inverter to handle your AC loads without triggering a low-voltage disconnect or melting a terminal lug.
The DC-to-AC Conversion Block: Source to Load
To understand how an inverter works DC to AC, you need to trace the power path from the battery terminals to your wall outlet. The system block flows in four distinct stages:
- DC Source & Bus Capacitor: Battery voltage feeds into a massive DC bus capacitor bank inside the inverter. This capacitor acts as a local energy reservoir, stabilizing the DC voltage and supplying the instantaneous high-current spikes required when the AC waveform crosses zero or hits peak voltage.
- H-Bridge Inverter Stage: The core of the conversion. Four electronic switches (arranged like the letter 'H') chop the DC bus voltage. By closing the top-left and bottom-right switches, current flows one way. By closing the top-right and bottom-left, current flows the opposite way. This creates the alternating polarity.
- PWM & LC Filter: A pure square wave will destroy modern electronics. The inverter's microcontroller uses high-frequency PWM (typically 20 kHz to 25 kHz) to vary the width of the DC pulses. An LC filter (an inductor and a capacitor) then averages these high-frequency pulses, smoothing them into a fluid sine wave.
- AC Output & Load: The filtered sine wave passes through an internal transfer relay (if it is an inverter/charger) and out to your AC panel or receptacle.
Battery Bank Architecture: Series vs. Parallel Consequences
Your inverter can only output what the DC source can deliver. When building a battery bank, you must choose between series and parallel wiring, which fundamentally changes your voltage (V) and amp-hour (Ah) capacity.
- Series Wiring (Voltage Adds, Ah Stays Same): Connecting four 12V 100Ah batteries in series yields a 48V 100Ah bank. The total energy is 4,800Wh. High voltage means lower current draw for the same wattage, allowing for thinner, cheaper copper wire between the battery and inverter.
- Parallel Wiring (Ah Adds, Voltage Stays Same): Connecting four 12V 100Ah batteries in parallel yields a 12V 400Ah bank. The total energy is still 4,800Wh. However, pulling 3,000W from a 12V bank requires over 250 amps of continuous current, demanding massive 4/0 AWG cables and posing severe thermal risks at the busbars.
When sizing your bank, you must account for Depth of Discharge (DoD) and C-rate limits. Lead-acid (AGM/Gel) batteries should not be discharged past 50% DoD if you want them to last more than a few hundred cycles. LiFePO4 batteries can safely be discharged to 80% or 90% DoD. Furthermore, most LiFePO4 server-rack batteries have a continuous discharge C-rate limit of 0.5C. A 100Ah battery is limited to 50A continuous output; pulling more will trip the BMS.
Sizing Math: Inverter, Charger, and Peukert’s Reality
Let's size an inverter and battery bank for a realistic off-grid cabin load: a continuous 2,500W draw with a 4,000W surge (for a well pump starting). We will use a 48V nominal LiFePO4 architecture.
1. Inverter Sizing and Efficiency Losses
Inverters are not 100% efficient. A high-quality unit like the Victron MultiPlus operates at about 90% efficiency at nominal load. To get 2,500W of AC output, the inverter must pull more from the DC side:
DC Input Power = AC Load / Efficiency = 2500W / 0.90 = 2,777W
At a nominal 48V (which is actually 51.2V for a fully charged 16-cell LiFePO4), the continuous DC current draw is:
DC Current = 2777W / 48V = 57.8 Amps
For the 4,000W surge, the instantaneous draw spikes to roughly 92 Amps. Therefore, you need an inverter rated for at least 3,000W continuous, and your battery BMS must support a 100A continuous discharge.
2. Peukert’s Law and Usable Capacity
If you were using Lead-Acid batteries, Peukert's Law would severely penalize you. Peukert’s law states that as the rate of discharge increases, the usable capacity of the battery decreases. An AGM battery with a Peukert exponent of 1.3 will lose nearly 40% of its rated capacity when discharged at a 1C rate. LiFePO4 batteries have a Peukert exponent very close to 1.05, meaning they deliver almost their full rated Ah capacity even under heavy loads. This is why lithium is mandatory for high-surge off-grid systems.
3. Charger Sizing
If your inverter includes an AC charger (to charge from a generator or grid), size it to replenish the bank in roughly 4 to 6 hours. A 35A to 50A charger is ideal for a 100Ah LiFePO4 bank, respecting the typical 0.5C max charge rate limit dictated by the BMS.
Decision Tree: Picking Your Inverter and Battery Topology
Use this decision path to lock in your system voltage and architecture based on your maximum continuous AC load.
| Max Continuous AC Load | System Voltage | Battery Architecture | Wire Gauge (Battery to Inverter) |
|---|---|---|---|
| Under 1,200W | 12V DC | 12V LiFePO4 (Parallel if needed) | 2/0 AWG Copper |
| 1,200W to 2,500W | 24V DC | 2x 12V in Series (24V) | 2/0 AWG Copper |
| 2,500W to 5,000W | 48V DC | 4x 12V in Series OR Native 48V Rack | 2/0 AWG or 1/0 AWG Copper |
| Over 5,000W | 48V DC (Split Phase) | Multiple 48V Racks in Parallel | 4/0 AWG Copper or Busbar |
The Default Build: 48V Victron and SOK Setup
Do not get paralyzed by analysis. If you are building a standard off-grid cabin, RV, or backup system with a 3,000W continuous load profile, here is the exact, field-tested bill of materials you should buy.
The Concrete Pick
- Inverter/Charger: Victron MultiPlus 48/3000/35-16 (Part Number: PMP482301102). This unit provides 3,000W continuous pure sine wave output, a 35A AC charger, and an integrated 16A transfer switch. It handles motor surges flawlessly.
- Battery Bank: SOK 48V 100Ah LiFePO4 Server Rack Battery (or equivalent 16S 48V 100Ah with a 100A BMS). This provides 5.12kWh of total energy. At 80% DoD, you have 4.09kWh of usable daily capacity.
- Main Cabling: 2/0 AWG pure copper welding cable for the positive and negative runs between the battery busbar and the inverter DC terminals. Keep this run under 5 feet to minimize voltage drop.
- Overcurrent Protection: A 125A Class T fuse (or MEGA fuse) mounted on the positive terminal of the battery bank, within 7 inches of the post, as required by NEC-style marine and off-grid standards.
- Communication: RJ45 CAT6 cable to link the battery BMS RS485 port to the Victron Cerbo GX or the inverter's VE.Bus port, ensuring the inverter automatically cuts off before the BMS triggers a hard low-voltage disconnect.
System Specifications Summary
| Nominal System Voltage | 48V DC (51.2V actual) |
| Continuous AC Output | 3,000W (VA rating: 3000VA) |
| Peak Surge Capability | 5,500W for 60 seconds |
| Usable Energy Capacity | 4.09 kWh (at 80% DoD) |
| Max Continuous DC Draw | ~70A (at full 3000W load) |
| BMS Discharge Limit | 100A Continuous |
By selecting a 48V architecture, you cut your DC current in half compared to a 24V system, drastically reducing heat generation at your terminal lugs and allowing the Victron's internal high-frequency transformers to operate at peak efficiency. Torque your DC terminal bolts to the manufacturer's spec (usually 10-12 Nm for M8 studs), apply a thin layer of dielctric grease to prevent oxidation, and your DC-to-AC conversion block will run cool and reliable for the next decade.
